Alkali vapor-cells have been used extensively since the 1960s in the study of light-atom interactions. Vapor-cell applications, both proposed and realized, include atomic clocks, communication system switches and buffers, single-photon generators and detectors, gas-phase sensors, nonlinear frequency generators, and precision spectroscopy instrumentation. However, most of these applications have only been created in laboratory settings.
A key driver has been to reduce vapor-cell size. Traditional vapor-cell systems are large and, if they have thermal control, have many discrete components and consume a large amount of power. To realize the full potential of vapor-cell technologies, the vapor-cell systems need to be miniaturized.
Macroscale vapor cells are widely used in macro-scale atomic clocks and as spectroscopy references. They are typically 10-100 cm3 in volume, which is insignificant for m3 scale atomic clocks, but is far too large for chip-scale atomic clocks which are at most a few cm3 in volume.
Chip-scale atomic clocks and navigation systems require miniature vapor cells, typically containing cesium or rubidium, with narrow absorption peaks that are stable over time. Miniature vapor cells, and methods of filling them with alkali metals, have been described in the prior art. However, it has proven difficult to load a precise amount of alkali metal and buffer gas into a miniature vapor cell through the methods described in the literature. Miniature vapor cells have higher surface-area-to-volume ratios than macro-scale vapor cells, and are more difficult to load than macroscale vapor cells.
Furthermore, once the alkali metal and buffer gas mixture is loaded into the vapor cell, the composition cannot be controlled. The only input that can be used to control the partial pressures of the alkali metal and of the buffer gas is temperature; however, temperature affects both partial pressures.
These known problems lead to a number of factors, including reverse reactions, helium diffusion into the vapor cell, and buffer and alkali diffusion out of the vapor cell. These factors contribute to insufficient long-term stability in miniature vapor cells.
There is a long-felt need for improved long-term frequency stability in vapor cells. What is desired, in particular, is independent control of the alkali vapor pressure in the vapor cell, i.e., independent from the system temperature.